Induction heating apparatuses have been employed for heating a variety of materials without direct contact therewith. For instance, heat treating of metals and melting of materials may be accomplished by induction heating. Further examples of induction heating applications include, without limitation, annealing, bonding, brazing, forging, stress relief, and tempering. Additionally, powder metallurgy applications may relate to heating of a mold or other member which, in turn, heats a powder metallurgy composition to be melted. Metal or other casting applications may also utilize induction heating. Accordingly, as known in the art, induction heating may be useful in various industries and applications.
For instance, one particular application for induction heating relates to treatment and storage of such hazardous materials and is known as “vitrification.” Hazardous materials may be vitrified when they are combined with glass forming materials and heated to relatively high temperatures. During vitrification, some of the hazardous constituents, such as hazardous organic compounds, may be destroyed by the high temperatures, or may be recovered as fuels. Other hazardous constituents, which are able to withstand the high temperatures, may form a molten state, which then cools to form a stable vitrified glass. The vitrified glass may demonstrate relatively high stability against chemical and environmental attack as well as a relatively high resistance to leaching, as by water, of the hazardous components contained therein.
One type of apparatus that has proven to be effective to vitrify waste materials is a cold-crucible-induction melter (CCIM). A cold-crucible-induction melter may typically comprise a water-cooled crucible disposed proximate to an induction coil or another inductor; for instance, an induction coil may be formed along a helical path extending about the crucible. Generally, an induction coil may carry alternating electric current that generates associated varying electromagnetic fields for inducing eddy currents within electrically conductive materials encountered thereby. The varying electromagnetic fields generated by the current within an inductor may be described as the “flux” thereof.
Waste may be induction heated directly if it is sufficiently electrically conductive and thus vitrified. However, the waste and glass forming materials used in vitrification systems may be relatively non-electrically conductive at room temperatures. Therefore, an electrically conductive material may be used to initially indirectly heat at least a portion of the waste to a molten state, at which point the waste may become more electrically conductive so that when varying current is conducted through the induction coil, conductive molten waste may be induction heated by way of eddy currents generated therein. Of course, non-electrically-conductive waste materials nearby the electrically conductive molten waste, due to the heat generated therein, may be indirectly heated and thus, melted.
As a further advantage of cold-crucible-induction melter vitrification systems, molten glass within the water-cooled crucible may form a solid layer (skull layer), which inhibits or prevents direct contact of the high temperature molten glass with the interior surface of the crucible. Furthermore, because the crucible itself is cooled with water, in combination with the insulative properties of the skull layer, relatively high-temperature melting may be achieved without being substantially limited by the heat-resistance or melting point of the crucible.
FIG. 1 shows a perspective view of a conventional induction melter 10. Generally, cold-crucible-induction melter 10 includes head assembly 20 affixed to disengagement spool 40 by way of mating lower flange 21 and upper flange 39 of head assembly 20 and disengagement spool 40, respectively. Disengagement spool 40 is affixed to furnace body 30 by way of lower flange 37, which is affixed to the upper flange 31 of the furnace body 30. Head assembly 20 includes off-gas port 12 for removing gasses from the cold-crucible-induction melter 10 during operation, feed port 14 for adding waste material to the cold-crucible-induction melter 10, and view port 15 for observing the conditions within the cold-crucible-induction melter 10. Furnace body 30 may include cooling tubes 22 disposed therearound, which may be supplied with a cooling medium, such as water, by way of inlet 23 and outlet 25 for cooling the crucible (not shown) and also includes bottom drain assembly (not shown) for discharging vitrified waste material from the crucible 56 (FIG. 2A) during operation of the cold-crucible-induction melter 10.
FIG. 2A shows a side cross-sectional view of the cold-crucible-induction melter 10 shown in FIG. 1. More particularly, an induction heating system 90 comprising an induction coil 26, a power source 100, and electrical conductors 110 extending therebetween may be configured for delivering heat to the interior of crucible 56. In further detail, induction heating system 90 may include an induction coil 26 disposed generally about the furnace body 30 of the cold-crucible-induction melter 10 as known in the art (cooling tubes 22 have been omitted from FIGS. 2A–2D for clarity). Both electrical conductors 110 and induction coil 26 may be water-cooled, as known in the art. Power source 100 may comprise a variable-frequency power supply, which is configured for energizing the induction coil 26 with a selectable, alternating electrical current having an amplitude and a frequency wherein at least one of the amplitude and frequency is variable. As known in the art, power source 100 may be operably coupled to or integrally inclusive of a capacitor bank (i.e., a plurality of capacitors) and a transformer, which are configured (separately or in combination) for tuning (automatically or manually) to the load (i.e., the material to be heated). Each of the plurality of capacitors may be configured to be individually and reversibly electrically coupled to the inductor via the controller.
FIG. 2B shows a side cross-sectional view of the cold-crucible-induction melter 10 shown in FIG. 1 (cooling tubes 22 have been omitted in FIG. 2B for clarity) including granular material 55, which may be disposed within crucible 56. For instance, granular material 55 may comprise hazardous materials and glass forming materials, without limitation. Also, susceptor 120 may be positioned in contact with the granular material 55 and may be configured for heating, in response to energizing induction coil 26, to a temperature sufficient to melt at least a portion of the granular material 55 proximate thereto. For instance, susceptor 120 may comprise graphite and may be shaped as a ring or as otherwise desired. The presence of a susceptor 120 may be necessary to initially melt at least a portion of the granular material 55, because the granular material 55 may not be electrically conductive in a non-molten state. Of course, conversely, if granular material 55 is electrically conductive in a non-molten state, susceptor 120 may be omitted as being unnecessary.
During initial operation of the induction heating system 90 of the cold-crucible-induction melter 10, as shown in FIG. 2B, assuming granular material 55 is not electrically conductive, induction coil 26 carrying an alternating current induces eddy currents within susceptor 120, thus heating susceptor 120. As susceptor 120 increases in temperature, granular material 55 proximate to susceptor 120 may be heated and may form a region of molten material 50 adjacent susceptor 120, as shown in FIG. 2C. Inductive heating by energizing induction coil 26 with an alternating current may then proceed by way of induced electrical currents within the molten material 50, assuming such molten material 50 becomes electrically conductive, in combination with heating of susceptor 120 by way of induced electrical currents therein until substantially the interior of crucible 56 comprises molten material 50, surrounded by skull layer 52, as explained further hereinbelow and shown in FIG. 2D.
Referring to FIG. 2D, granular material 55 may be introduced within cold-crucible-induction melter 10 through feed port 14 and ultimately melted to form molten material 50, which may substantially fill crucible 56. Susceptor 120 (FIGS. 2B and 2C) may be sacrificial, and may substantially oxidize (burn off) or may break into several pieces within molten material 50. As noted previously, crucible 56 may be surrounded by cooling tubes 22 for flowing water or gas through in order to cool the crucible 56 during operation, because the temperatures that may be required to vitrify waste materials may exceed the melting point of the crucible 56. The desired steady-state operational temperature for vitrifying waste material may be about 1200° Celsius. Cooling the crucible 56 during heating of the waste may form a skull layer 52 comprising solidified material (previously molten material 50) disposed along the inner surface of the side wall of the crucible 56. The skull layer 52 may be from a few millimeters to several inches in thickness, and may insulate the molten material 50 within the crucible 56 and also inhibit the molten material 50 from directly contacting and damaging the inner surface of the crucible 56. Skull layer 52 may span a relatively extreme temperature gradient between the cooling water temperature within cooling tubes 22, which may be less than about 100° Celsius, and the molten material 50 temperature, which may be greater than about 1000° Celsius. Of course, the relative thickness of the skull layer 52 may vary depending on the thermal environment of the crucible 56.
Also, cold cap 54, comprising granular material 55 and, possibly, condensed off-gas material, may preferably exist upon the upper surface of molten material 50 thereof under preferred conditions. Cold cap 54 may reduce volatilization of molten material 50 and may also insulate molten material 50. Impact zone 59 indicates a region of cold cap 54 that granular material 55, shown as entering the cold-crucible-induction melter 10 through feedport 14, may fall upon and accumulate. Dust, volatized material, and evolved gases 57 may exit or move upwardly away from the impact zone 59 of cold cap 54 into the plenum volume 200. Ultimately, dust, volatized material, and evolved gases 57 may subsequently condense, deposit, or settle onto cold cap 54, adhere to the inner wall of disengagement spool 40 or head assembly 20, respectively, or exit the plenum volume 200 through offgas port 12.
Induction coils 26 surrounding crucible 56 may be energized with relatively large alternating currents to induce currents within the waste material to be heated. Typically, induction coils 26 may be fabricated from a highly electrically conductive material, such as copper, and are cooled by water or another fluid flowing therein. As known in the art, waste materials, such as radioactive waste or other waste may be combined with glass forming constituents, heated, and thereby vitrified.
Generally, conventional induction heating systems may be configured for heating in response to a temperature set-point. More particularly, conventional induction heating systems may be configured for varying the output power of the power source 100 in relation to the difference between a desired temperature and a measured temperature of the material to be heated. However, while such a temperature feedback control system may be relatively effective in controlling the temperature, it may not be particularly electrically efficient. Put another way, the transmission of electrical power between the induction coil 26 and the material that is heated therewith (e.g., the molten material 50, the susceptor 120, etc.) may be relatively inefficient.
Further, there may be difficulties in obtaining reliable temperature information relating to the molten material 50 that may complicate operation of the cold-crucible-induction melter 10. Therefore, conventional cold-crucible-induction melters may be often controlled manually. For example, conventional cold-crucible-induction melters may be controlled by “feel” or by secondary indications such as so-called “frequency pulling” in relation to the applied frequency of an induction power source 100. Such methods of control may be even more electrically inefficient than temperature feedback methods, and may also promote unintended variances from a desired temperature due to operator errors.
One approach for operating an induction melting furnace for glass (i.e., a cold-crucible-induction melter), disclosed by U.S. Pat. No. 6,185,243 to Boen et al., includes a melting furnace, including a cooled crucible having continuous metal side walls, a partitioned and cooled bottom and at least one induction coil positioned under the bottom of the crucible. The at least one induction coil is disclosed to be the sole heating means for materials within the crucible. The depth of the melting bath contained in the crucible and the excitation frequency of the induction coil are selected so that the depth and half of the inside radius of the crucible are less than the skin thickness of the bath.
In view of the foregoing problems and shortcomings with existing induction heating processing materials and systems, it would be advantageous to provide control methods relating to increased efficiency for operation of cold-crucible-induction melters.